1. Field of the Invention
The present invention relates generally to processes and apparatuses for the pasteurization of materials, and more particularly to a method and device for killing microorganisms on food materials such as raw meat, fruits and vegetables through controlled application of a biocidal treatment gas into a subatmospheric environment.
2. Description of the Prior Art
Prior to slaughter, the muscles of healthy food animals normally do not contain microorganisms toxic to humans. The animal's gastrointestinal tract, will however harbor several species of microorganisms. Typically, the host of gastrointestinal microorganisms found in animals will include numerous species of bacteria. Enterobacteria such as Salmonella is especially prevalent. While these microorganisms are generally not harmful to the animal, several species will produce toxic reactions in humans.
Unfortunately, an essential part of slaughter is the cutting and removal of an animal's gastrointestinal tract. Not surprisingly, the tract's contents are often spilled and smeared onto the meat surface during and after slaughter. Further contributing to bacterial contamination, gastrointestinal tract contents are often spread from the surface of one animal to another as the result of successive handling by slaughterhouse workers. Contamination can also occur during successive machine-processing steps and when meat pieces are sequentially dipped in various liquid treatment baths. Generally, surface-to-surface contamination can occur throughout every stage in any standard meat slaughtering, processing and packaging assembly-line.
As pointed out in Busch et al. (U.S. Pat No. 3,934,044) many techniques have been employed in the past for treating meat and meat cuts in order to destroy surface bacterial flora. As noted by Busch et al. however, all of these techniques have suffered from one or more of the following problems: (1) denaturation of meat protein, (2) insufficient bacterial kill, (3) deleterious color change, (4) unacceptable flavor modification, and (5) inadequate control of the process in large scale operations.
The problems enumerated by Busch et al. are evident in several other previous and subsequent patents. For example, Malkki et al. (U.S. Pat. No. 3,996,386) attempts to prevent microbial surface deterioration of foods by spraying the surface of such foods with a preservative in the form of a fine particulate aerosol. Suggested preservatives include chlortetracycline, chloramphenicol, sodium o-phenyl phenolate and others. While this process provides a means for countering surface bacterial growth it still results in a chemically coated food substance despite efforts to minimize actual amounts of preservatives used. Accordingly, potential exists for chemical interactions leading to undesirable color change and/or flavor modification.
In the same vein, Robinson et al. (U.S. Pat. No. 4,636,395) describes a method for heat treating the surface area of raw meats by rapidly elevating the temperature of the meat followed immediately by rapid cooling. Although Robinson et al. does not use taste and/or color modifying preservatives, their process is delicately balanced between the competing objectives of maximizing sufficient bacterial kill and preventing the denaturation of meat protein. It is believed that this balance struck by Robinson et al. is one that can be consistently maintained only with difficulty and constant vigilance due to its having very few safeguards. In actual practice of the Robinson et al. technology one objective will often have to give way to the other.
The Busch et al. process (U.S. Pat. No. 3,934,044) attempts to correct the several noted problems. Busch et al. describes a process directed at destroying psychotropic spoilage bacteria on meat or meat-cuts without adversely affecting the color, flavor or aroma of the meat by applying a hot, dilute acid solution to meat surfaces for a brief period of time. However, in the attempt to correct the inherent problems associated with the pasteurization of meat products, Busch et al. overlooks the problem of surface irregularities, that were identified by Malkki et al. in their later patent.
These irregularities, in the form of pores and other imperfections found on the surface of meat, will often serve as fertile grounds for the development of bacterial growth. The removal of surface contamination from meat through use of conventional pasteurization procedures has been further frustrated by the strong adhesion of microorganisms to the meat surface. When a wash or spray is used, many organisms tend to escape. While better results were achieved with the addition of bactericides to the spray or wash water, the improvement was slight. Even with the use of concentrated and powerful significant number of organisms still often survived. This occurred even when the exposure time and bactericide concentration were more than adequate to sterilize, a smooth, simple surface.
In accordance with the invention of Morgan (U.S. Pat. No. 5,281,428, hereby incorporated by reference) the failure of prior processes could be ascribed to chemical and physical attributes of the solvent, water. Generally, water cannot reach deep contaminated surfaces because of its high surface tension. Many other solvents used for microbial kill exhibit similarly high surface tension. The folds, cavities and pores are often large enough to contain bacteria; but too small to admit a liquid wash or spray--with this resulting in the need for use of an impractically high wash pressure to overcome the capillary pressure in pores having dimensions just large enough to contain bacteria.
Many species of microorganisms are small relative to the surface irregularities in which they live. Many parts of the muscles are about the same size as the typical target organism, 2.times.10.sup.-6 m. Poultry muscle fibers, for example, are about 20.times.10.sup.-6 m. in diameter; and these each consist of three distinct sheaths, covering many of the much finer fibrils. These fibrils are generally of two kinds: thick and thin. The thick fibrils are 0.1.times.10.sup.-6 m. wide and 2.times.10.sup.-6 m. long. The thin fibrils are 0.05.times.10.sup.-6 m. wide and 0.4.times.10.sup.-6 m. long. In addition, there are several other structures, such as Z discs, mitochondria, capillaries, and cell nuclei in the muscles, capable of obscuring the targets. Furthermore, certain toxic bacteria, such as Salmonella, have short flagella on their surface which may entangle with the meat fibers or fibrils. In view of this, one can appreciate the difficulties inherent in cleaning meat products.
With these issues in mind, Morgan developed and patented an apparatus and process involving a requisite 4-step treatment for the killing of the microorganisms on porously surfaced materials such as meat. These steps require the sequential use of a first vacuum, a flush, a gas treatment and a second vacuum.
It is in light of the precedent invention of Morgan, that the present invention was designed. In the most general sense, the present invention provides a means to kill microorganisms on both porous and non-porous material surfaces without damaging the material itself and without need for a flushing step--thus increasing the effective speed of treatment. None of the patents discussed above, taken either singly or in combination, describe or suggest the instant invention as claimed.